An Active Matrix Organic Light Emitting Diode (AMOLED) drives an Organic Light Emitting Diode (OLED) to emit light by using Thin Film Transistors (TFTs).
A driving manner of an OLED pixel circuit may be classified as a current-driving manner and a voltage-driving manner, and in a voltage-driving circuit, a current IOLED flowing through the OLED may be calculated with an Equation as follows:
      I    OLED    =            1      2        ⁢                  μ        n            ·      Cox      ·              W        L            ·                        (                                    V              data                        -                          V              OLED                        -                          V              th                                )                2            
herein, μn is a mobility of carriers, Cox is a capacitance in an oxide layer at a gate,
  W  Lis a width-length ratio of the transistor, Vdata is a data voltage, VOLED is an operational voltage of the OLED and is shared by all pixel units, Vth is a threshold voltage of the transistor, which is a positive value for an enhanced TFT and is a negative value for a depletion TFT.
It can be seen from the above equation that the current would be different if the Vth is different among the different pixel units. If the Vth of a pixel drifts as time elapses, the currents before and after drifting would be different and the image sticking may occur. Also, the differences in the current may also be caused by differences in the operational voltages of the OLEDs due to non-uniformity in the OLED devices.
The current-driving mode is advantageous over the voltage-driving mode in that, the current IOLED=Idata, and a current-driving circuit would have a function for adjusting a level of the present current by itself if the threshold voltage of the pixel drifts as time elapses, which is independent of the Vth of TFTs, and a display which is uniform spatially and is stable temporally would be realized. However, the current-driving circuit is generally applied to a screen with a small size because of its long driving time.
FIG. 1 is a diagram illustrating a circuit structure of the existing current-driving manner. An operation of this circuit is divided into two periods: a pre-charging period and a light-emitting period. In the first period, a power supply ARVDD of the pixel circuit is at a low level, a transistor T4 is turned off, a scan signal SCAN is at a high level, transistors T1 and T2 are turned on, and a capacitor Cs is charged; in the second period, the power supply ARVDD of the pixel circuit is at the high level, the scan signal SCAN is at the low level, the transistors T1 and T2 are turned off, and an OLED emits light. Such a current-driving pixel circuit has a defect of over-long charging time, which thus limits the application scope of the current-driving pixel circuit.